An antenna whose primary gain lobe can be electronically steered in two dimensions is desirable in many applications. In the prior art the two dimensional steering is most commonly provided by phased array antennas. Phased array antennas have complex electronics and are quite costly.
In the prior art, various electronically steered artificial impedance surface antennas (AISAs) have been described that have one dimensional electronic steering, including the AISAs described in U.S. Pat. Nos. 7,245,269, 7,071,888, and U.S. Pat. No. 7,253,780 to Sievenpiper. These antennas are useful for some applications, but are not suitable for all applications that need two dimensional steering. In some applications mechanical steering can be used to provide steering of a 1D electronically steered antenna in a second dimension. However, there are many applications where mechanical steering is very undesirable. The antennas described by Sievenpiper also require vias for providing voltage control to varactors.
A two dimensionally electronically steered AISA has been described in U.S. Pat. No. 8,436,785, issued on May 7, 2013, to Lai and Colburn. The antenna described by Lai and Colburn is relatively costly and is electronically complex, because to steer in two dimensions a complex network of voltage control to a two dimensional array of impedance elements is required so that an arbitrary impedance pattern can be created to produce beam steering in any direction.
Artificial impedance surface antennas (AISAS) are realized by launching a surface wave across an artificial impedance surface (AIS), whose impedance is spatially modulated across the AIS according a function that matches the phase fronts between the surface wave on the AIS and the desired far-field radiation pattern.
In previous references, listed below, references [1]-[6] describe artificial impedance surface antennas (AISA) formed from modulated artificial impedance surfaces (AIS). Patel [1] demonstrated a scalar AISA using an end-fire, flare-fed one-dimensional, spatially-modulated AIS consisting of a linear array of metallic strips on a grounded dielectric. Sievenpiper, Colburn and Fong [2]-[4] have demonstrated scalar and tensor AISAs on both flat and curved surfaces using waveguide- or dipole-fed, two-dimensional, spatially-modulated AIS consisting of a grounded dielectric topped with a grid of metallic patches. Gregoire [5]-[6] has examined the dependence of AISA operation on its design properties.
Referring to FIG. 1, the basic principle of AISA operation is to use the grid momentum of the modulated AIS to match the wave vectors of an excited surface-wave front to a desired plane wave. In the one-dimensional case, this can be expressed asksw=ko sin θo−kp  (1)
where ko is the radiation's free-space wavenumber at the design frequency, θo is the angle of the desired radiation with respect to the AIS normal, kp=2π/p is the AIS grid momentum where p is the AIS modulation period, and ksw=noko is the surface wave's wavenumber, where no is the surface wave's refractive index averaged over the AIS modulation. The SW impedance is typically chosen to have a pattern that modulates the SW impedance sinusoidally along the SWG according toZ(x)=X+M cos(2πx/p)  (2)
where p is the period of the modulation, X is the mean impedance, and M is the modulation amplitude. X, M and p are chosen such that the angle of the radiation θ in the x-z plane w.r.t the z axis is determined byθ=sin−1(n0−λ0/p)  (3)
where n0 is the mean SW index, and λ0 is the free-space wavelength of radiation. n0 is related to Z(x) by
                              n          0                =                                            1              p                        ⁢                                          ∫                0                p                            ⁢                                                                    1                    +                                                                  Z                        ⁡                                                  (                          x                          )                                                                    2                                                                      ⁢                                                                  ⁢                                  ⅆ                  x                                                              ≈                                    1              +                              X                2                                                                        (        4        )            
The AISA impedance modulation of Eqn. (2) can be generalized for an AISA of any shape asZ({right arrow over (r)})=X+M cos(konor−{right arrow over (k)}o□{right arrow over (r)})  (5)
where {right arrow over (k)}o is the desired radiation wave vector, {right arrow over (r)} is the three-dimensional position vector of the AIS, and r is the distance along the AIS from the surface-wave source to {right arrow over (r)} along a geodesic on the AIS surface. This expression can be used to determine the index modulation for an AISA of any geometry, flat, cylindrical, spherical, or any arbitrary shape. In some cases, determining the value of r is geometrically complex.
For a flat AISA, it is simply r=√{square root over (x2+y2)}.
For a flat AISA designed to radiate into the wave vector at {right arrow over (k)}=ko(sin θo{circumflex over (x)}+cos θo{circumflex over (z)}), with the surface-wave source located at x=y=0, the modulation function isZ(x,y)=X+M cos(ko(nor−x sin θo))  (6)
The cos function in Eqn. (2) can be replaced with any periodic function and the AISA will still operate as designed, but the details of the side lobes, bandwidth and beam squint will be affected.
The AIS can be realized as a grid of metallic patches on a grounded dielectric. The desired index modulation is produced by varying the size of the patches according to a function that correlates the patch size to the surface wave index. The correlation between index and patch size can be determined using simulations, calculation and/or measurement techniques. For example, Colburn [3] and Fong [4] use a combination of HFSS unit-cell eigenvalue simulations and near field measurements of test boards to determine their correlation function. Fast approximate methods presented by Luukkonen [7] can also be used to calculate the correlation. However, empirical correction factors are often applied to these methods. In many regimes, these methods agree very well with HFSS eigenvalue simulations and near-field measurements. They break down when the patch size is large compared to the substrate thickness, or when the surface-wave phase shift per unit cell approaches 180°.
In the prior art electronically-steerable AIS antennas described in [8] and [9], the AIS is a grid of metallic patches on a dielectric substrate. The surface-wave impedance is locally controlled at each position on the AIS by applying a variable voltage to voltage-variable varactors connected between each of the patches. It is well known that an AIS's SW impedance can be tuned with capacitive loads inserted between impedance elements [8], [9]. Each patch is electrically connected to neighboring patches on all four sides with voltage-variable varactor capacitor. The voltage is applied to the varactors though electrical vias connected to each impedance element patch. Half of the patches are electrically connected to the groundplane with vias that run from the center of each patch down through the dielectric substrate. The rest of the patches are electrically connected to voltage sources that run through the substrates, and through holes in the ground plane to the voltage sources.
Computer control allows any desired impedance pattern to be applied to the AIS within the limits of the varactor tunability and the AIS SW property limitations. One of the limitations of this method is that the vias can severely reduce the operation bandwidth of the AIS because the vias also impart an inductance to the AIS that shifts the SW bandgap to lower frequency. As the varactors are tuned to higher capacitance, the AIS inductance is increased and this further reduces the SW bandgap frequency. The net result of the SW bandgap is that it does not allow the AIS to be used above the bandgap frequency. It also limits the range of SW impedance that the AIS can be tuned to.